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The ability of bone marrow derived cells to contribute to skeletal muscle repair may represent a novel means of cell therapy for myopathies. However, this phenomenon takes place at exceedingly low frequencies and has failed to yield any measurable functional improvement in disease models. In an effort to increase the efficiency of this process, we designed Cre/loxP-based tracing strategies to identify the lineages and mechanisms involved. However, these experiments were complicated by a previously unknown limitation of common Cre-reporter strains.
We have studied the Z/AP, Z/EG and R26R-EYFP reporter strains and have demonstrated that although each reporter can be reliably activated by Cre during early development, exposure to Cre in adult hematopoietic cells results in a much lower frequency of reporter-positive cells in the Z/AP or Z/EG strains than in the R26R-EYFP strain. In reporter-negative cells derived from the Z/AP and Z/EG strains, the transgenic promoter is methylated and Cre-mediated recombination of the locus is inhibited. These findings suggest that the Z/AP and Z/EG strains may not be suitable for the investigation of developmental plasticity in adult models.
As bone marrow derived cells are now believed to contribute to skeletal muscle repair primarily via fusion, we also constructed a chimeric measles hemagglutinin, Hα7, which efficiently mediates the fusion of diverse cell types with skeletal muscle. When compared directly to polyethylene glycol in vitro, Hα7 consistently generated a ten to fifteen fold increase in heterokaryon yield and induced insignificant levels of toxicity. More importantly, Hα7 was also capable of increasing the contribution of mouse and human bone marrow derived cells to skeletal muscle repair in vivo.